Aircraft bus systems, in the general sense, are not new. For years, different applications have demanded bus systems that supply power for use by aircraft equipment. In recent years, such bus systems have been made more flexible to allow external sources to provide power to the bus system to permit engine starting via a motor/generator, and, in the case of an aircraft having more than one channel, such bus systems also have included connections between the channels to allow one channel to supply power to the other for cross-channel engine starting.
On a modern jet aircraft, it is important to have a bus system which is flexible, fault tolerant and efficient. Flexibility in today's aircraft means that the bus system must be able to accept power from a number of power sources, including jet engine-driven variable speed constant frequency solid state power supplies, auxiliary power units, ram air driven emergency power generators and ground cart power supplies. Furthermore, true flexibility includes the ability not only to deliver high quality three phase 400 Hertz power to sensitive electronic equipment, but to deliver three phase variable frequency power to an engine-starting motor, thereby eliminating the need to start the engine with conventional means (either an airframe mounted or cart mounted air turbine starter). This engine-starting capability also has utility when the plane is flying; if an engine stalls, no cart would be present.
Fault tolerance is also a critical issue. As aircraft grow more complex, a greater chance arises for faults to occur within the electrical system. If the bus system is not able to work around these faults at least partially as they occur, the effect upon the airplane's continued ability to fly could be devastating. Accordingly, aircraft are now provided with dual channel redundant bus systems, at a minimum. Therefore, at least some equipment should remain functional in the event of bus feeder or relay failure. Failure is a concern given the extreme operating conditions under which these systems must function. Electrical buses must span the length of the aircraft, hundreds of feet in some cases. Cross-start feeders must span from engine to engine, which may be a hundred feed through wing, wing root and fuselage. Structural failure of the airplane may sever or short a bus or feeder. Relays must remain mechanically operable in the face of extreme heat, cold and vibration for hundreds of hours. The bus system must allow for failure of any one of the several relays in the typical system. In short, improvements in bus systems tend to yield remarkable increases in flexibility and bus reliability.
Finally, bus systems must be efficient. Since the power buses and feeders are so long, line losses due to resistance and stray inductance and capacitance become significant. In power-critical modes of operation (such as when a converter in one channel supplies power to a converter in another channel), line losses must be kept to a minimum. Otherwise, generators and converters must be designed larger and heavier to compensate for the extra load brought about by line losses. Furthermore, since converters are only between 80% and 90% efficient, it is desirable to avoid using two converters in series, which only further increases power requirements.
Several patents address the feature of providing a bus system which operates as a power generating system in one mode and an engine starting system in another mode.
Typical of such patents is U.S. Pat. No. 4,330,743, which issued on May 18, 1982 to Glennon, entitled "Electrical Aircraft Engine Start and Generating System" and assigned to the assignee of the instant invention. The Glennon invention is directed to an electrical aircraft engine start and generating system for use in an aircraft having an engine driven torque converter coupled to an alternator which provides AC power for conversion to CC and AC power. The system includes a reversible AC to DC converter controllably electrically coupled to the alternator and a controller unit to provide DC power in a generating mode. The reversible AC to DC converter is capable of receiving externally supplied DC power to be converted to AC power to drive the alternator as a motor in a start mode. A DC to AC converter is controllably electrically coupled to the controller unit and the DC power output during the generating mode. The reversible DC to AC converter in the start mode is mutually controllably electrically coupled to the externally supplied DC power. The controller unit and the alternator cooperate to provide a controlled AC power output to be delivered to the alternator to bring the alternator operating as a motor up to operating speed, whereupon the reversible AC to DC converter responds to the external DC power and to the electrically coupled alternator to drive the alternator as a motor to deliver rotary power through the torque converter to start the aircraft engine. Accordingly, Glennon provides for a bidirectional system: one which provides constant frequency power to aircraft loads by converting variable frequency power supplied by a prime mover via a motor/generator and one into which power may be accepted from an external power source and delivered to the prime mover via the motor/generator as motive power to start the prime mover. Glennon describes the operation of a single channel system for starting an engine using DC electric power as the power source. Although Glennon does deal with electrical engine starting, apparently it does not deal at all with a dual channel cross-start configuration. The instant invention concerns the feeder and relay configuration in and between the buses of a dual channel aircraft power system.
Also typical of such aircraft power systems is that described in U.S. Pat. No. 4,481,459, which issued on Nov. 6, 1984 to Mehl et al., entitled "Combined Starting/Generating System and Method" and assigned to the assignee of the instant invention.
Mehl et al. is directed to a power conversion system for converting between electric and motive power which may be utilized either in a generating mode to generate electric power from motive power supplied by a prime mover or in a starting mode wherein motive power is developed by the power conversion system from electrical power and is supplied through a torque converter to the prime mover to start the same. The power conversion system includes a main generator, an exciter and a permanent magnet generator, or PMG, which together comprise a brushless alternator. When operated in the starting mode, power is supplied to the PMG to cause it to act as a motor and thereby drive a rotor which is common to the PMG, exciter and main generator. Once a predetermined operating condition of the generator is attained, the main generator is supplied power from a motor control to cause the generator to act as a synchronous motor and the power supply to the PMG is disconnected. The torque converter is then commanded to transfer motive power from the generator to the prime mover to start same. Apparently, among other things, Mehl et al. describes the operation of a start system that uses AC electrical power as the power source as contrasted with Glennon which discloses an engine starting system using DC power as the power source. As with Glennon, Mehl et al. deals with electrical engine starting in a single channel system. However, Mehl et al. evidently does not deal at all with a dual channel configuration having cross-start capability.
Still other patents have dealt with the problem of providing parallel power sources with interconnections therebetween to allow transfer of power from one source to another to provide for an uninterruptable power supply.
U S. Pat. No. 4,645,940, which issued on Feb. 24, 1987 to Wertheim, entitled "Interrupt-Free Unregulated Power Supply" is an example. Wertheim is directed to a primary and at least one back-up alternating voltage source coupled to a common load to provide interrupt-free electric current through a simplified interconnect network. Phase control of all but one of the sources is accomplished by monitoring the phase difference across choke coils coupling the sources to the load. In other words, Wertheim provides for multiple interconnected sources, but fails to provide for engine starting, a primary object of the instant invention. Further, Wertheim is not directed to a variable speed constant frequency bus system.
In the past, need has arisen for dual channel power systems which provide for generate, direct start and cross-start modes of operation. There are two prior art dual channel power systems, not known to be patented, which provide for variable speed constant frequency power generation and direct and cross-starting of aircraft engines.
Reference is now made to FIG. 1 which shows, diagrammatically, an existing dual bus scheme with cross-start capability (designated "System I"). Reference is also made to FIG. 2, which diagrammatically shows another existing dual bus scheme with cross-start capability (designated "System II").
Both System I and System II will be compared to the instant invention and differences noted below since a fuller appreciation of the advantages of the instant invention is best obtained by showing the operation of all three bus configurations under a variety of operating conditions.
None of the aforementioned prior art inventions provide for cross-start operation wherein one motor/generator, operating as a generator operates the other motor/generator as a motor via a single converter, thereby increasing the efficiency of power transmission. Further, none of the aforementioned inventions was designed to overcome the problem of isolating faults in cross-start feeders and relays in order to maintain power generation, direct start and cross-start operation in the face of the cross-start feeder or relay fault. The instant invention is the first to provide a variable speed constant frequency bus system which provides for single converter generator-to-motor cross-start operation and the first to address the problem of isolating cross-start faults, providing for normal channel operation during generate mode.